SYNOPSIS

DESCRIPTION

Packet sockets are used to receive or send raw packets at the device
driver (OSI Layer 2) level. They allow the user to implement protocol
modules in user space on top of the physical layer.
The socket_type is either SOCK_RAW for raw packets including the link
level header or SOCK_DGRAM for cooked packets with the link level
header removed. The link level header information is available in a
common format in a sockaddr_ll. protocol is the IEEE 802.3 protocol
number in network order. See the <linux/if_ether.h> include file for a
list of allowed protocols. When protocol is set to htons(ETH_P_ALL)
then all protocols are received. All incoming packets of that protocol
type will be passed to the packet socket before they are passed to the
protocols implemented in the kernel.
Only processes with effective UID 0 or the CAP_NET_RAW capability may
open packet sockets.
SOCK_RAW packets are passed to and from the device driver without any
changes in the packet data. When receiving a packet, the address is
still parsed and passed in a standard sockaddr_ll address structure.
When transmitting a packet, the user supplied buffer should contain the
physical layer header. That packet is then queued unmodified to the
network driver of the interface defined by the destination address.
Some device drivers always add other headers. SOCK_RAW is similar to
but not compatible with the obsolete AF_INET/SOCK_PACKET of Linux 2.0.
SOCK_DGRAM operates on a slightly higher level. The physical header is
removed before the packet is passed to the user. Packets sent through
a SOCK_DGRAM packet socket get a suitable physical layer header based
on the information in the sockaddr_ll destination address before they
are queued.
By default all packets of the specified protocol type are passed to a
packet socket. To only get packets from a specific interface use
bind(2) specifying an address in a structsockaddr_ll to bind the
packet socket to an interface. Only the sll_protocol and the
sll_ifindex address fields are used for purposes of binding.
The connect(2) operation is not supported on packet sockets.
When the MSG_TRUNC flag is passed to recvmsg(2), recv(2), recvfrom(2)
the real length of the packet on the wire is always returned, even when
it is longer than the buffer.
AddressTypes
The sockaddr_ll is a device independent physical layer address.
struct sockaddr_ll {
unsigned short sll_family; /* Always AF_PACKET */
unsigned short sll_protocol; /* Physical layer protocol */
int sll_ifindex; /* Interface number */
unsigned short sll_hatype; /* Header type */
unsigned char sll_pkttype; /* Packet type */
unsigned char sll_halen; /* Length of address */
unsigned char sll_addr[8]; /* Physical layer address */
};
sll_protocol is the standard ethernet protocol type in network order as
defined in the <linux/if_ether.h> include file. It defaults to the
socket’s protocol. sll_ifindex is the interface index of the interface
(see netdevice(7)); 0 matches any interface (only permitted for
binding). sll_hatype is a ARP type as defined in the <linux/if_arp.h>
include file. sll_pkttype contains the packet type. Valid types are
PACKET_HOST for a packet addressed to the local host, PACKET_BROADCAST
for a physical layer broadcast packet, PACKET_MULTICAST for a packet
sent to a physical layer multicast address, PACKET_OTHERHOST for a
packet to some other host that has been caught by a device driver in
promiscuous mode, and PACKET_OUTGOING for a packet originated from the
local host that is looped back to a packet socket. These types make
only sense for receiving. sll_addr and sll_halen contain the physical
layer (e.g., IEEE 802.3) address and its length. The exact
interpretation depends on the device.
When you send packets it is enough to specify sll_family, sll_addr,
sll_halen, sll_ifindex. The other fields should be 0. sll_hatype and
sll_pkttype are set on received packets for your information. For bind
only sll_protocol and sll_ifindex are used.
SocketOptions
Packet sockets can be used to configure physical layer multicasting and
promiscuous mode. It works by calling setsockopt(2) on a packet socket
for SOL_PACKET and one of the options PACKET_ADD_MEMBERSHIP to add a
binding or PACKET_DROP_MEMBERSHIP to drop it. They both expect a
packet_mreq structure as argument:
struct packet_mreq {
int mr_ifindex; /* interface index */
unsigned short mr_type; /* action */
unsigned short mr_alen; /* address length */
unsigned char mr_address[8]; /* physical layer address */
};
mr_ifindex contains the interface index for the interface whose status
should be changed. The mr_type parameter specifies which action to
perform. PACKET_MR_PROMISC enables receiving all packets on a shared
medium (often known as "promiscuous mode"), PACKET_MR_MULTICAST binds
the socket to the physical layer multicast group specified in
mr_address and mr_alen, and PACKET_MR_ALLMULTI sets the socket up to
receive all multicast packets arriving at the interface.
In addition the traditional ioctls SIOCSIFFLAGS, SIOCADDMULTI,
SIOCDELMULTI can be used for the same purpose.
IoctlsSIOCGSTAMP can be used to receive the timestamp of the last received
packet. Argument is a structtimeval.
In addition all standard ioctls defined in netdevice(7) and socket(7)
are valid on packet sockets.
ErrorHandling
Packet sockets do no error handling other than errors occurred while
passing the packet to the device driver. They don’t have the concept
of a pending error.

NOTES

For portable programs it is suggested to use AF_PACKET via pcap(3);
although this only covers a subset of the AF_PACKET features.
The SOCK_DGRAM packet sockets make no attempt to create or parse the
IEEE 802.2 LLC header for a IEEE 802.3 frame. When ETH_P_802_3 is
specified as protocol for sending the kernel creates the 802.3 frame
and fills out the length field; the user has to supply the LLC header
to get a fully conforming packet. Incoming 802.3 packets are not
multiplexed on the DSAP/SSAP protocol fields; instead they are supplied
to the user as protocol ETH_P_802_2 with the LLC header prepended. It
is thus not possible to bind to ETH_P_802_3; bind to ETH_P_802_2
instead and do the protocol multiplex yourself. The default for
sending is the standard Ethernet DIX encapsulation with the protocol
filled in.
Packet sockets are not subject to the input or output firewall chains.
Compatibility
In Linux 2.0, the only way to get a packet socket was by calling
socket(AF_INET,SOCK_PACKET,protocol). This is still supported but
strongly deprecated. The main difference between the two methods is
that SOCK_PACKET uses the old structsockaddr_pkt to specify an
interface, which doesn’t provide physical layer independence.
struct sockaddr_pkt {
unsigned short spkt_family;
unsigned char spkt_device[14];
unsigned short spkt_protocol;
};
spkt_family contains the device type, spkt_protocol is the IEEE 802.3
protocol type as defined in <sys/if_ether.h> and spkt_device is the
device name as a null-terminated string, for example, eth0.
This structure is obsolete and should not be used in new code.

BUGS

glibc 2.1 does not have a define for SOL_PACKET. The suggested
workaround is to use:
#ifndef SOL_PACKET
#define SOL_PACKET 263
#endif
This is fixed in later glibc versions and also does not occur on libc5
systems.
The IEEE 802.2/803.3 LLC handling could be considered as a bug.
Socket filters are not documented.
The MSG_TRUNCrecvmsg(2) extension is an ugly hack and should be
replaced by a control message. There is currently no way to get the
original destination address of packets via SOCK_DGRAM.